During inspection of an electrical circuit (like Printed Circuit Boards, semiconductor wafers, etc.), high resolution photography is used, for detection of defects. An electrical circuit is usually placed on a mechanical stage (hereinafter-stage), that moves in respect to a camera that obtains images of the electrical circuit.
Two major illumination schemes are known and include a continuous acquisition and illumination (e.g. using a line scan camera or a time-delay integration—TDI—camera), a snap-shot acquisition and flash illumination.
The flash illumination has to be synchronized to the camera snap-shot to prevent acquiring an at least partly black image (or at least a non well illuminated picture).
Also, it is usually required to know where the image has been captured relative to the moving stage. Prior art system stages include a stage encoder that issues a trigger upon an arrival of the stage to a desired location, respective to a location where an inspected circuit (or the stage itself) should be located for a proper image to be acquired.
Conveniently, such a trigger will trigger the flash and the camera for illumination and acquisition respectively. Also, it is noted that flash duration should usually be relatively short so the image will not be blurred during motion.
FIG. 1 illustrates a prior art system 100 and various signals that are sent between components of the system 100.
System 100 includes stage 101, stage encoder 110, controller 170, distributer (also referred to as communication board 120), strobe illumination unit (such as flash 130), camera 140, frame grabber 150 and personal computer (PC) 160.
Stage 101 carries the electrical circuits to a designated direction (X-axis, Y-axis or both X-axis and Y-axis movement) at a speed that varies and is not maintained at a fixed value.
The camera 140 has a rectangular field of view (frame) that includes FOVx×FOVy pixels. FOVx is the number of pixels along the x-axis and FOVy is the number of pixels along the y-axis.
Stage encoder 110 is connected to stage 101 and issues a pulse (trigger) that is relative to a distance or a movement of the stage 10 (e.g. every FOVy*PIXEL_SIZEy, in microns). This stage movement based trigger activates the camera 140 to acquire images, and triggers the strobe illumination unit 130 to illuminate the electrical circuit.
The images which are captured by camera 140 in response to such triggering are transmitted to a frame grabber 150 and then are sent to PC 160 for image processing.
As indicated above, the frequency of the triggering signal (of a camera trigger and/or a flash trigger) is initiated by the position of the stage in different times.
Frame grabber 150 can acquire images at a fixed maximal rate. Temporal speed increments of stage 101 can temporarily increase the provision rate of images by frame grabber 150 as stage 101 passes the distance of FOVy*PIXEL_SIZEy more quickly. In order to manage these temporal speed increments, stage 101 should move the electrical circuit at a speed that is lower than an optimal speed.
In addition, system 100 must acquire images while maintaining a fixed overlap between consecutive images of areas of the electrical circuit. This demand requires system 100 to impose strict limitation on the speed of stage 101 and move stage 101 slower than maximal speed to provide better controllability.
FIG. 2 illustrates the signals that are being exchanged between the components of system 100.
At time Tp stage encoder 110 sends a triggering signal “position” that is responsive to the location of stage 101 on which an inspected electrical circuit is placed. The “position” triggering signal is sent to controller 170 that, in response, sends a “start grab” triggering signal to distributer 120. The distributer 120 sends a “grab” triggering signal to frame grabber 150 and sends a “flash” triggering signal to strobe illumination unit 130. The “flash” triggering signal is sent in a delay τ1 after Tp. Frame grabber 150 receives the “grab” triggering signal and sends an “open shutter” triggering signal to camera 140.
The “grab” triggering signal is sent to frame grabber 150 before the “open shutter” triggering signal is sent to camera 140. The “open shutter” signal precedes the “flash” triggering signal that is sent to the strobe illumination unit 130.
The “open shutter” triggering signal causes camera 140 to obtain an image and send it (as indicated by the “image” arrow) to frame grabber 150. Frame grabber 150 sends the image to computer 160.
The duration τ1 which is the latency between the time that the stage is in a known position and the time in which the strobe flashes could usually be calibrated, but the flashing of the flash/strobe in the prior art inspection is substantially triggered by a location of the stage.